1. Field of the Invention
This invention pertains to an air spring impact mechanism for a sledgehammer and/or hammer drill.
2. Description of the Related Art
Of the common types of impact mechanisms in use today for hammer drills or sledgehammers, one design among others has been proven effective in which a drive piston is moved by a crankshaft back and forth inside of a percussion piston, also called a hollow impactor. The percussion piston in turn moves axially inside of an impact mechanism housing, resulting in the formation of an air spring between the drive piston and the percussion piston. This spring conveys the axial motion of the drive piston onto the percussion piston and propels it against a tool shank or an anvil where the momentum of the percussion piston is converted to an impact.
One impact mechanism of this type, for example as seen in DE-OS 22 60 365, has quite a few advantages in comparison to other types. For one thing, the relatively light drive piston produces only minimal vibrations during idling. Furthermore, the impact mechanism is of a simple design, keeping the manufacturing costs low and making it easy to repair. In addition, the impact mechanism has good cold-starting characteristics as it heats up rapidly due to the grease friction between the drive piston and the percussion piston.
However, these types of impact mechanisms are difficult to design with regard to their reverse suction behavior and idle behavior. In particular, if the impact mechanism is designed so that after a successful impact the drive piston always reliably pulls back the percussion piston by suction, the transition to idle mode can be rough when the tool is lifted off of the rock being worked on. In this case, there is the danger that the percussion piston can continue to issue impacts in its idle position. These so-called idle impacts are very unpleasant for the operator and put a large stress on the hammer. On the other hand, if the impact mechanism is designed to provide a reliable transition to idle, the risk arises that the percussion piston is not always being retracted by suction by the drive piston, even during impact operation. This can result in an interruption of impact operation, and thus to irregular functioning.
Another type of impact mechanism is known from DE-OS 22 62 273 in which a percussion piston and a drive piston with the same diameter, both of which move axially, are arranged in a common guide tube. There are idle openings in the guide tube that the percussion piston can pass over when the tool is lifted off of the rock being worked on. This allows the cavity between the percussion piston and the drive piston to communicate with the surroundings, preventing a return suction effect by the drive piston and keeping the percussion piston in its prior position.
From DE 196 21 057 A1, an air spring impact mechanism is known in which the impact by the percussion piston can be dampened by an additional air spring located in front of the percussion piston.
In DE 23 35 924 A1, an air spring impact mechanism is described with an impact mechanism housing, a hollow percussion piston with guide sleeve and a drive piston that moves inside the guide sleeve. Between the drive piston and the percussion piston is a cavity. Also, an air equalization pocket is provided on the inside of the guide sleeve of the percussion piston for the purpose of filling the cavity with air. Idle operation is not possible in the air spring impact mechanism shown. In fact, the drive motor must be shut off to interrupt the work.
The objective of this invention is to provide an air spring impact mechanism with a hollow percussion piston in which the disadvantages described above are avoided without having to forfeit the advantages mentioned.
According to the invention, the objective is met by an air spring impact mechanism as specified in patent claim 1.
In the air spring impact mechanism according to the invention, the inside of a guide sleeve of the percussion piston that forms the cavity thereof is provided with at least one air equalization pocket that extends in the axial direction. Furthermore, at least one idle opening passes through the guide sleeve radially. During impact operation, no connection exists between the idle opening and an idle channel in the impact mechanism housing that leads to the surroundings. This isolates from the surroundings the cavity between the drive piston and the percussion piston constituting the air spring itself On the other hand, during idle operation the idle opening is allowed to pass over the idle channel so that this cavity is connected to the surroundings by means of the idle opening and the idle channel. This enables intensive air equalization during idling, preventing any suction effects caused by the drive piston.
It is preferred for the air equalization pocket to have a larger axial length than a surface of the drive piston in contact with the guide sleeve. This allows air leaking from the air spring with each impact to be refilled from the air equalization pocket.
In an especially advantageous embodiment of the invention, there are a number of idle openings in the guide sleeve that can be arranged in an axially offset fashion. Moreover, the idle openings can have differing cross sectional areas. Depending on the arrangement of the idle openings and on their cross sections, it is possible to design the transition between idle and impact operation to be smooth, which the operator will find particularly comfortable. Also, the tool can be placed at exposed points without the tool jumping inadvertently.
Another embodiment of the invention consists of providing the guide sleeve with numerous glide surfaces upon which it is guided within the impact mechanism housing. Between each of the glide surfaces are notches with smaller radii. As a result of the reduction in frictional surface, and thus reduced friction, between the percussion piston and the impact mechanism housing, better cold-starting behavior can be achieved at low temperatures. Furthermore, the manufacturing costs can be reduced because of the reduction in exterior surface that needs to be machined. The idle openings should be arranged such that they each penetrate a glide surface since this prevents the need for any additional sealing between the guide sleeve of the percussion piston and the impact mechanism housing.
In another advantageous embodiment of the invention, an air cushion is produced in a front cavity between a front side of the percussion piston and the impact mechanism housing. When the front cavity is temporarily isolated from the surroundings as the percussion piston transitions from impact operation to idle operationxe2x80x94in other words when the percussion piston moves forward in the direction of the idle positionxe2x80x94air pressure can build up in this cavity so that some of the kinetic energy of the percussion piston is conveyed to the air spring arising in the cavity.
In another advantageous embodiment of the invention, when the percussion piston has reached its final idle position, the front cavity can be brought to connect to the surroundings through one of the notches. This allows the air compressed in the front cavity to leak out, releasing the energy stored in the air spring by pressure relief.
The result, then, is that the air spring also functions as an air cushion, decreasing the kinetic energy of the percussion piston so that it is braked before it hits an end wall of the housing and thus reaches its idle position. The percussion piston is thus prevented from impacting the housing too heavily, which is also a comfort to the operator.